In a cellular type telecommunications system, it is known that terminal calls occupying a cell or a site are relayed by means of a base station. In accordance with the UMTS standard, the term “cell” is used to designate a geographical zone having a single carrier frequency band allocated thereto, while the term “site” designates a geographical zone having a plurality of carrier frequency bands and/or a plurality of sectors, generally adjacent bands and/or sectors allocated thereto. In which case, it is the general practice to use a single power amplifier for all of the carrier frequencies of a sector, with the amplifier then being referred to as a “multicarrier amplifier”. More generally, in order to minimize the number of kinds of equipment that need to be produced, it is preferable to use a transmitter of the same type (same power class), and a power amplifier of the same type in all of the sectors of all sites.
A power amplifier of that type must satisfy a certain number of contradictory requirements; in particular, the amplifier must conserve its properties of linearity over a broad range while nevertheless providing high energy efficiency.
A power amplifier for transmitting a plurality of adjacent carrier frequencies presents a high peak-to-average power ratio (PAR). The higher this ratio the lower the efficiency of the amplifier. To remedy that drawback, it is known to limit the peak power of the combined signal before applying it to the input of the amplifier. However such peak clipping spoils the signal and gives rise to distortion in the band under consideration, and also outside said band(s). To comply with standards concerning out-of-band pollution, the peak clipping is therefore limited, i.e. the increase in efficiency that can be obtained using that technique is limited.
Because of those various constraints, it is not generally possible to use the same type of power amplifier system, i.e. the same transmitter, for an extensive site having relatively low subscriber density such as a cell in a rural zone with a single frequency band, and for one or more sites having significantly higher densities of users, commonly using a plurality of adjacent frequency bands.
In particular, if an operator seeks to use a single type of transmission equipment for a “rural” cell with a single frequency band and for one or more “urban” sites having N, e.g. 3, adjacent frequency bands, even if an efficient peak-clipping method is used, the transmission power for the rural cell is restricted to N times the power per carrier for the other sites. Because of the attenuation factor due to propagation, which is about 2.5, if the operator seeks to provide a high speed data service in a rural cell with a single frequency band, the radius of said rural cell is less than twice the radius of a high-traffic urban cell.